The substrate of an ultra-high speed IC package housing a semiconductor element, such as GaAs, Ge, Si, or the like, must have high mechanical strength, provide adequate electrical insulation, and provide an effective gas seal. Heretofore, to meet the aforementioned requirements, ceramic materials such as alumina and beryllia have been used to form such substrates. To join these substrates to metal conductors, terminal portions have been provided on the surface of the substrates and the metal conductors have been brazed to the terminal portions with an Ag or Ag-Cu eutectic brazing filler material at temperatures above 800.degree. C.
IC packages having ceramic substrates with high dielectric constants such as alumina and beryllia, however, suffer from the disadvantage that when the IC package is used in a high frequency band on the order of several giga hertz (GHz), the signal propagation speed is limited and, thus, the IC package cannot be used in such higher frequency bands. To overcome this limitation, it has been proposed in Japanese patent application (OPI) No. 92943/1984 to form the ceramic substrate of the IC package from a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO--ZnO-based sintered crystallized glass. It has been found, however, that because such crystallized glass substrates are lower in mechanical strength and heat resistance than alumina and beryllia, the use of Ag-based brazing filler materials causes cracks at the junction of the terminal portion of the substrate and the metal conductor due to the difference in thermal expansion during brazing.
To overcome the aforementioned cracking problem, it has been proposed in Japanese Utility Model Application No. 30456/1985 to braze metal conductors to the terminal portions of such substrates in IC packages using an Au-Sn or Au-Si eutectic brazing filler material at temperatures below 400.degree. C. Such brazing filler materials allow metal conductors to be readily brazed to the terminal portions of the ceramic substrate and the thus formed IC package displays excellent characteristics in higher frequency bands. The use of such brazing filler materials, however, suffers from the disadvantage that the sealing temperature for housing the semiconductor elements is in the range of from 400.degree. C. to 500.degree. C. and the thus formed junctions are low in heat resistance.
Accordingly, it is an object of the invention to provide a method of connecting a metal conductor to a ceramic substrate which overcomes the difficulties and disadvantages of the prior art.
Additional objects will be set forth in part in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention.